It is frequently necessary to heat one of the gaseous products of an air separation device to a temperature of use. In particular, it is known:                to use an electric or steam heater to heat the residual nitrogen from a cold box to regenerate the adsorbents of an air purification unit upstream of the cold box;        to preheat the oxygen injected into an oxycombustion boiler with flue gases.        
Using electricity to heat a fluid amounts to wasting “noble” energy because the efficiency of conversion between thermal energy and electrical energy does not exceed 50% at best.
In a power station, bleeding steam from the steam cycle can lead to significant electricity production losses.
From a thermodynamic point of view, in FIG. 1, it is seen that the exchange diagram representing the exchange of heat E on the abscissa axis and the temperature T on the ordinate axis for heating residual nitrogen WN2 with steam V is pinched at the hot end, but has a high ΔT at the cold end. Even recovering heat from the condensates of the steam (there would then be a lower ΔT at the cold end), the exchange diagram would remain overall very spread out (i.e. the area between the curves remains very large, which signifies high entropic loss).
In an “oxycombustion” type power station, for preheating oxygen sent to the oxycombustion process:                the flue gases from the boiler may be used;        the overall efficiency of the installation may be improved by recovering heat at the outlet of the compressors of the air separation device.        
In the latter case, a gas/gas (air/O2) exchanger may be used, but this is a very large piece of equipment that necessitates a large exchange area, whilst having a very low head loss.